Thanks for the reply Sarah!
He hasn't been tested for mono. He hasn't had a fever and his tiredness could also be because he doesn't go to bed early enough. I don't think he has pus at the back of his throat and it didn't even look irritated until this past week. I'll try to get him to go back to the doctor and ask for a referal to an ENT or allergist. It's weird that it seems to come and go throughout the day.